A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake system of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, comprises a solenoid actuator that is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, evaporative emission space that is cooperatively defined primarily by the tank headspace and the canister is purged to the engine intake system through the canister purge valve. For example, fuel vapors may be purged to an intake manifold of an engine intake system by the opening of a CPS-type valve in response to a signal from the engine management computer, causing the valve to open in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace, and/or stored in the canister, for entrainment with combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure.
It is believed fair to say that from a historical viewpoint two basic types of vapor leak detection systems for determining integrity of an evaporative emission space have evolved: a positive pressure system that performs a test by positively pressurizing an evaporative emission space; and a negative pressure (i.e. vacuum) system that performs a test by negatively pressurizing (i.e. drawing vacuum in) an evaporative emission space. The former may utilize a pressurizing device, such as a pump, for pressurizing the evaporative emission space; the latter may utilize either a devoted device, such as a vacuum pump, or engine manifold vacuum created by running of the engine.
Commonly owned U.S. Patents and Patent Applications disclose various systems, devices, modules, and methods for performing evaporative emission leak detection tests by positive and negative pressurization of the evaporative emission space being tested. Commonly owned U.S. Pat. No. 5,383,437 discloses the use of a reciprocating pump that alternately executes a downstroke and an upstroke to create positive pressure in the evaporative emission space. Commonly owned U.S. Pat. No. 5,474,050 embodies advantages of the pump of U.S. Pat. No. 5,383,437 while providing certain improvements in the organization and arrangement of a reciprocating pump.
The pump comprises a housing having an interior that is divided by a movable wall into a pumping chamber to one side of the movable wall and a vacuum chamber to the other side. One cycle of pump reciprocation comprises a downstroke followed by an upstroke. During a downstroke, a charge of air that is in the pumping chamber is compressed by the motion of the movable wall, and a portion of the compressed charge is expelled through a one-way valve, and ultimately into the evaporative emission space being tested. The movable wall moves in a direction that contracts the pumping chamber volume while expanding the vacuum chamber volume, and the prime mover for the downstroke motion is a mechanical spring that is disposed within the vacuum chamber to act on the movable wall. During a downstroke, the spring releases stored energy to move the wall and force air through the one-way valve. At the end of a downstroke, further compression of the air charge ceases, and so the consequent lack of further compression prevents the one-way valve from remaining open.
During an upstroke, the movable wall moves in a direction that expands the volume of the pumping chamber, while contracting that of the vacuum chamber. During the upstroke, the one-way valve remains closed, but a pressure differential is created across a second one-way valve causing the latter valve to open. Atmospheric air can then flow through the second valve to enter the pumping chamber. At the end of an upstroke, a charge of air has once again been created in the pumping chamber, and at that time, the second valve closes due to lack of sufficient pressure differential to maintain it open. The pumping mechanism can then again be downstroked.
The upstroke motion of the movable wall increasingly compresses the mechanical spring to restore the energy that was released during the immediately preceding downstroke. Energy for executing an upstroke is obtained from a vacuum source, intake manifold vacuum in particular. During an upstroke a solenoid valve operates to a condition that communicates the vacuum chamber of the pump to manifold vacuum. The vacuum is strong enough to have moved the movable wall to a position where, at the end of an upstroke, the pumping chamber volume is at a maximum and that of the vacuum chamber is at a minimum. A downstroke is initiated by operating the solenoid valve to a condition that vents the vacuum chamber to atmosphere. With loss of vacuum in the vacuum chamber, the spring can be effective to move the movable wall on a downstroke.
Operation of the solenoid valve to its respective conditions is controlled by a suitable sensor or switch that is disposed in association with the pump to sense when the movable wall has reached the end of a downstroke. When the sensor or switch senses the end of a downstroke, it delivers, to an associated controller, a signal that is processed by the controller to operate the solenoid valve to communicate vacuum to the vacuum chamber. The controller operates the solenoid valve to that condition long enough to assure full upstroking, and then it operates the solenoid to vent the vacuum chamber to atmosphere so that the next downstroke can commence. At the beginning of a downstroke, the pumping chamber holds a know volume of air at atmospheric pressure. The pump is a displacement pump that has a uniform swept volume, meaning that it displaces a uniform volume of air from the pumping chamber on each full downstroke. The mass of air displaced during each full downstroke is uniform, but as the pressure in the space being tested increases, the air must be compressed to progressively increasing pressure. Because the pumping chamber contains the same known volume of air at the same known pressure at the beginning of each downstroke, and because the stroke is well defined, the time duration of the downstroke correlates with pressure in the space being tested.
The pumping mechanism is repeatedly stroked in the foregoing manner as the test proceeds. Assuming that there is no gross leak that prevents the pressure from increasing toward a nominal test pressure suitable for obtaining a leak measurement, the amount of time required to execute a downstroke becomes increasingly longer as the nominal test pressure is approached. For an evaporative emission space that has zero leakage, the pressure will eventually reach the nominal test pressure, and pump stroking will cease when that occurs. For an evaporative emission space that has small leakage less than a gross leak, the pressure will stabilize substantially at the nominal test pressure, but the pump will continue stroking because it is continually striving to make up for the leakage that is occurring. The duration of the pump downstroke is indicative of the effective leak size, and that duration decreases with increasing effective leak size. Decreasing time duration of the pump downstroke means that the pump is stroking at increasing frequency, and hence a correlation between effective leak size and pump stroke frequency also exists. Therefore, a measurement of the time interval from the end of one downstroke, as sensed by the previously mentioned sensor or switch, until the end of the immediately following downstroke, as sensed by the sensor or switch, yields a substantially accurate measurement of effective leak size. Stated another way, the rate at which the pump cycles, i.e. strokes, is indicative of effective leak size once nominal test pressure has been reached.
The accuracy of this type of test is premised on substantially constant volume of the test space and on an ability to attain nominal test pressure stability. An ability to attain nominal test pressure stability within a reasonable period of time may be a factor in minimizing the total test time, and commercial acceptance of such leak detection systems may be conditioned on accomplishing a test in fairly short overall test time. It is therefore considered desirable for stability of nominal test pressure to be promptly achieved. Because change in the size of a leak during a test would affect test accuracy, it is understood that a test result is valid only when such a change does not occur during a test.
It has been observed however that the environment of an automotive vehicle may be hostile to promptly reaching nominal test pressure stability. To some extent, the nature of the test itself may also be responsible. The pump's compression of air is not an adiabatic process, and therefore, the compression also heats the air that is being pumped into the evaporative emission space. The added heat will inherently dissipate over time to the surroundings, but as it does, there is corresponding decrease in pressure as required by physical phenomena embodied in known gas laws. Hence, for a given leak indication system of this type in a vehicle, it appears that physical laws establish some minimum time interval for attaining nominal test pressure stability, thereby precluding the shortening of that interval below that minimum.